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+;;; edit-indirect.el --- Edit regions in separate buffers -*- lexical-binding: t -*-
+
+;; Author: Fanael Linithien <fanael4@gmail.com>
+;; URL: https://github.com/Fanael/edit-indirect
+;; Version: 0.1.10
+;; Package-Requires: ((emacs "24.3"))
+
+;; This file is NOT part of GNU Emacs.
+
+;; SPDX-License-Identifier: BSD-2-clause
+;;
+;; Copyright (c) 2014-2022, Fanael Linithien
+;; All rights reserved.
+;;
+;; Redistribution and use in source and binary forms, with or without
+;; modification, are permitted provided that the following conditions are
+;; met:
+;;
+;;   * Redistributions of source code must retain the above copyright
+;;     notice, this list of conditions and the following disclaimer.
+;;   * Redistributions in binary form must reproduce the above copyright
+;;     notice, this list of conditions and the following disclaimer in the
+;;     documentation and/or other materials provided with the distribution.
+;;
+;; THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
+;; IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+;; TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+;; PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
+;; OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+;; EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+;; PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+;; PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+;; LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+;; NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+;; SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+;;; Commentary:
+
+;; Edit regions in separate buffers, like `org-edit-src-code' but for arbitrary
+;; regions.
+;;
+;; See the docstring of `edit-indirect-region' for details.
+
+;;; Code:
+(defgroup edit-indirect nil
+  "Editing regions in separate buffers."
+  :group 'editing)
+
+(defcustom edit-indirect-guess-mode-function #'edit-indirect-default-guess-mode
+  "The function used to guess the major mode of an edit-indirect buffer.
+It's called with the edit-indirect buffer as the current buffer.
+It's called with three arguments, the parent buffer, the beginning
+and the end of the parent buffer region being editing.
+
+Note that the buffer-local value from the parent buffer is used."
+  :type 'function
+  :group 'edit-indirect)
+
+(defcustom edit-indirect-after-creation-hook nil
+  "Functions called after the edit-indirect buffer is created.
+The functions are called with the edit-indirect buffer as the
+current buffer.
+
+Note that the buffer-local value from the parent buffer is used."
+  :type 'hook
+  :group 'edit-indirect)
+
+(defcustom edit-indirect-before-commit-hook nil
+  "Functions called before the edit-indirect buffer is committed.
+The functions are called with the edit-indirect buffer as the
+current buffer.
+
+Note that the buffer-local value from the edit-indirect buffer is
+used."
+  :type 'hook
+  :group 'edit-indirect)
+
+(defcustom edit-indirect-before-commit-functions nil
+  "Functions called before an edit-indirect buffer is committed.
+The functions are called with the parent buffer as the current
+buffer.
+Each function is called with two arguments, the beginning and the
+end of the region to be changed."
+  :type 'hook
+  :group 'edit-indirect)
+
+(defcustom edit-indirect-after-commit-functions nil
+  "Functions called after an edit-indirect buffer has been committed.
+The functions are called with the parent buffer as the current
+buffer.
+Each function is called with two arguments, the beginning and the
+end of the changed region."
+  :type 'hook
+  :group 'edit-indirect)
+
+(defgroup edit-indirect-faces nil
+  "Faces used in `edit-indirect'."
+  :group 'edit-indirect
+  :group 'faces
+  :prefix "edit-indirect")
+
+(defface edit-indirect-edited-region
+  '((t :inherit secondary-selection))
+  "Face used to highlight an indirectly edited region."
+  :group 'edit-indirect-faces)
+
+;; Emacs <= 24.3 has no `define-error'.
+(let* ((user-error-conditions (get 'user-error 'error-conditions))
+       (define-user-error (lambda (name message)
+                            (put name 'error-conditions
+                                 (cons name user-error-conditions))
+                            (put name 'error-message message))))
+  (funcall define-user-error 'edit-indirect-overlapping
+           "Indirectly edited regions cannot overlap")
+  (funcall define-user-error 'edit-indirect-read-only
+           "Text is read-only, modify the edit-indirect buffer instead")
+  (funcall define-user-error 'edit-indirect-not-indirect
+           "This is not an edit-indirect buffer"))
+
+(defvar edit-indirect--overlay)
+(defvar edit-indirect--should-quit-window nil)
+(put 'edit-indirect--should-quit-window 'permanent-local t)
+
+;;;###autoload
+(defun edit-indirect-region (beg end &optional display-buffer)
+  "Edit the region BEG..END in a separate buffer.
+The region is copied, without text properties, to a separate
+buffer, called edit-indirect buffer, and
+`edit-indirect-guess-mode-function' is called to set the major
+mode.
+When done, exit with `edit-indirect-commit', which will remove the
+original region and replace it with the edited version; or with
+`edit-indirect-abort', which will drop the modifications.
+
+This differs from `clone-indirect-buffer' with narrowing in that
+the text properties are not shared, so the parent buffer major mode
+and the edit-indirect buffer major mode will not be able to tread
+on each other's toes by setting up potentially conflicting text
+properties, which happens surprisingly often when the font-lock
+mode is used.
+
+Edit-indirect buffers use the `edit-indirect-mode-map' keymap.
+Regions with active edit-indirect buffers use the
+`edit-indirect-overlay-map' keymap.
+
+If there's already an edit-indirect buffer for BEG..END, use that.
+If there's already an edit-indirect buffer active overlapping any
+portion of BEG..END, an `edit-indirect-overlapping' error is
+signaled.
+
+When DISPLAY-BUFFER is non-nil or when called interactively,
+display the edit-indirect buffer in some window and select it.
+
+In any case, return the edit-indirect buffer."
+  (interactive
+   (if (or (use-region-p) (not transient-mark-mode))
+       (prog1 (list (region-beginning) (region-end) t)
+         (deactivate-mark))
+     (user-error "No region")))
+  (let ((buffer (edit-indirect--get-edit-indirect-buffer beg end)))
+    (when display-buffer
+      (edit-indirect--display-buffer buffer))
+    buffer))
+
+(defvar edit-indirect-mode-map
+  (let ((map (make-sparse-keymap)))
+    (define-key map [remap save-buffer] #'edit-indirect-save)
+    (define-key map (kbd "C-c '") #'edit-indirect-commit)
+    (define-key map (kbd "C-c C-c") #'edit-indirect-commit)
+    (define-key map (kbd "C-c C-k") #'edit-indirect-abort)
+    map)
+  "Keymap for edit-indirect buffers.
+
+\\{edit-indirect-mode-map}")
+
+(defvar edit-indirect-overlay-map
+  (let ((map (make-sparse-keymap)))
+    (define-key map (kbd "RET") #'edit-indirect-display-active-buffer)
+    map)
+  "Keymap for regions with active edit-indirect buffers.
+
+\\{edit-indirect-overlay-map}")
+
+(defun edit-indirect-commit ()
+  "Commit the modifications done in an edit-indirect buffer.
+That is, replace the original region in the parent buffer with the
+contents of the edit-indirect buffer.
+The edit-indirect buffer is then killed.
+
+Can be called only when the current buffer is an edit-indirect
+buffer."
+  (interactive)
+  (edit-indirect--barf-if-not-indirect)
+  (edit-indirect--commit)
+  (edit-indirect--clean-up))
+
+(defun edit-indirect-save ()
+  "Save the modifications done in an edit-indirect buffer.
+That is, replace the original region in the parent buffer with the
+contents of the edit-indirect buffer.
+
+Can be called only when the current buffer is an edit-indirect
+buffer."
+  (interactive)
+  (edit-indirect--barf-if-not-indirect)
+  (edit-indirect--commit))
+
+(defun edit-indirect-abort ()
+  "Abort indirect editing in the current buffer and kill the buffer.
+
+Can be called only when the current buffer is an edit-indirect
+buffer."
+  (interactive)
+  (edit-indirect--barf-if-not-indirect)
+  (edit-indirect--abort))
+
+(defun edit-indirect-buffer-indirect-p (&optional buffer)
+  "Non-nil iff the BUFFER is an edit-indirect buffer.
+BUFFER defaults to the current buffer."
+  (save-current-buffer
+    (when buffer
+      (set-buffer buffer))
+    ;; (not (null)) so we don't leak the overlay to the outside world.
+    (not (null edit-indirect--overlay))))
+
+(defun edit-indirect-default-guess-mode (_parent-buffer _beg _end)
+  "Guess the major mode for an edit-indirect buffer.
+It's done by calling `normal-mode'."
+  (normal-mode))
+
+(defun edit-indirect-display-active-buffer ()
+  "Display the active edit-indirect buffer of the region the point is in."
+  (interactive)
+  (let ((overlay
+         (let ((p (point)))
+           (edit-indirect--search-for-edit-indirect p (1+ p)))))
+    (unless overlay
+      (signal 'edit-indirect-not-indirect '()))
+    (edit-indirect--display-buffer (overlay-get overlay 'edit-indirect-buffer))))
+
+(defvar edit-indirect--overlay nil
+  "The overlay spanning the region of the parent buffer being edited.
+
+It's also used as the variable determining if we're in an
+edit-indirect buffer at all.")
+(make-variable-buffer-local 'edit-indirect--overlay)
+(put 'edit-indirect--overlay 'permanent-local t)
+
+;; Normally this would use `define-minor-mode', but that makes the mode function
+;; interactive, which we don't want, because it's just an implementation detail.
+(defun edit-indirect--mode (overlay)
+  "Turn the `edit-indirect--mode' \"minor mode\" on.
+OVERLAY is the value to set `edit-indirect--overlay' to."
+  (setq edit-indirect--overlay overlay)
+  (add-hook 'kill-buffer-hook #'edit-indirect--abort-on-kill-buffer nil t))
+(with-no-warnings
+  (add-minor-mode
+   'edit-indirect--overlay " indirect" edit-indirect-mode-map nil #'ignore))
+
+(defun edit-indirect--display-buffer (buffer)
+  "Display the given BUFFER in some window and select it."
+  (with-current-buffer buffer
+    (setq-local edit-indirect--should-quit-window t))
+  (select-window (display-buffer buffer))
+  nil)
+
+(defun edit-indirect--get-edit-indirect-buffer (beg end)
+  "Return an edit-indirect buffer for the region BEG..END.
+If there's already an edit-indirect buffer active overlapping any
+portion of BEG..END, an `edit-indirect-overlapping' error is
+signaled."
+  (let ((old-overlay (edit-indirect--search-for-edit-indirect beg end)))
+    (cond
+     ((null old-overlay)
+      (let ((overlay (edit-indirect--create-overlay beg end)))
+        (edit-indirect--create-indirect-buffer beg end overlay)))
+     ((and (= beg (overlay-start old-overlay))
+           (= end (overlay-end old-overlay)))
+      (overlay-get old-overlay 'edit-indirect-buffer))
+     (t
+      (signal 'edit-indirect-overlapping '())))))
+
+(defun edit-indirect--search-for-edit-indirect (beg end)
+  "Return an existing edit-indirect overlay for some region inside BEG..END.
+If there's no indirectly edited region inside BEG..END, return
+nil."
+  (catch 'done
+    (dolist (overlay (overlays-in beg end))
+      (when (overlay-get overlay 'edit-indirect-buffer)
+        (throw 'done overlay)))
+    nil))
+
+(defmacro edit-indirect--buffer-local-value (buffer variable)
+  "Get the BUFFER local value of VARIABLE.
+VARIABLE shall be a symbol."
+  (unless (symbolp variable)
+    (signal 'wrong-type-argument (list #'symbolp variable)))
+  ;; `with-current-buffer' is used instead of `buffer-local-value' because
+  ;; the latter doesn't give warnings about free variables when
+  ;; byte-compiled.
+  `(with-current-buffer ,buffer ,variable))
+
+(defun edit-indirect--create-indirect-buffer (beg end overlay)
+  "Create an edit-indirect buffer and return it.
+
+BEG..END is the parent buffer region to insert.
+OVERLAY is the overlay, see `edit-indirect--overlay'."
+  (add-hook 'after-change-major-mode-hook #'edit-indirect--rebind-save-hooks)
+  (let ((buffer (generate-new-buffer (format "*edit-indirect %s*" (buffer-name))))
+        (parent-buffer (current-buffer)))
+    (overlay-put overlay 'edit-indirect-buffer buffer)
+    (with-current-buffer buffer
+      (insert-buffer-substring-no-properties parent-buffer beg end)
+      (set-buffer-modified-p nil)
+      (edit-indirect--mode overlay)
+      ;; Use the buffer-local values from the parent buffer. Don't retrieve the
+      ;; values before actual uses in case these variables are changed by some
+      ;; of the many possible hooks.
+      (funcall (edit-indirect--buffer-local-value
+                parent-buffer edit-indirect-guess-mode-function)
+               parent-buffer beg end)
+      (if (local-variable-p 'edit-indirect-after-creation-hook parent-buffer)
+          ;; Copy the parent buffer hook to the indirect buffer instead of
+          ;; let-binding it to avoid running it twice.
+          (setq-local edit-indirect-after-creation-hook
+                      (edit-indirect--buffer-local-value
+                       parent-buffer edit-indirect-after-creation-hook))
+        ;; No need to do copy anything if the parent buffer has no local value,
+        ;; the global value will be used instead. Just kill the local value in
+        ;; the indirect buffer in case a prior hook set it, because we're not
+        ;; supposed to use it.
+        (kill-local-variable 'edit-indirect-after-creation-hook))
+      (run-hooks 'edit-indirect-after-creation-hook))
+    buffer))
+
+(defun edit-indirect--create-overlay (beg end)
+  "Create the edit-indirect overlay and return it.
+
+BEG and END specify the region the overlay should encompass."
+  (let ((overlay (make-overlay beg end)))
+    (overlay-put overlay 'face 'edit-indirect-edited-region)
+    (overlay-put overlay 'modification-hooks '(edit-indirect--barf-read-only))
+    (overlay-put overlay 'insert-in-front-hooks '(edit-indirect--barf-read-only))
+    (overlay-put overlay 'keymap edit-indirect-overlay-map)
+    overlay))
+
+(defvar edit-indirect--inhibit-read-only nil
+  "Non-nil means disregard read-only status of indirectly-edited region.")
+
+(defun edit-indirect--barf-read-only (_ov _after _beg _end &optional _len)
+  "Signal an error because the text is read-only.
+No error is signaled if `inhibit-read-only' or
+`edit-indirect--inhibit-read-only' is non-nil."
+  (unless (or inhibit-read-only edit-indirect--inhibit-read-only)
+    (signal 'edit-indirect-read-only '())))
+
+(defun edit-indirect--commit ()
+  "Commit the modifications done in an edit-indirect buffer."
+  (run-hooks 'edit-indirect-before-commit-hook)
+  (let ((beg (overlay-start edit-indirect--overlay))
+        (end (overlay-end edit-indirect--overlay))
+        (buffer (current-buffer))
+        (edit-indirect--inhibit-read-only t))
+    (with-current-buffer (overlay-buffer edit-indirect--overlay)
+      (save-excursion
+        (let ((beg-marker (copy-marker beg))
+              (end-marker (copy-marker end)))
+          (edit-indirect--run-hook-with-positions
+           'edit-indirect-before-commit-functions beg-marker end-marker)
+          (save-match-data
+            (set-match-data (list beg-marker end-marker))
+            (let ((new-data
+                   (with-current-buffer buffer
+                     (buffer-substring-no-properties 1 (1+ (buffer-size))))))
+              (unless (string= new-data (match-string 0))
+                (replace-match new-data t t))))
+          (edit-indirect--run-hook-with-positions
+           'edit-indirect-after-commit-functions beg-marker (point))
+          (set-marker beg-marker nil)
+          (set-marker end-marker nil))))
+    (set-buffer-modified-p nil)))
+
+(defun edit-indirect--run-hook-with-positions (hook beg end)
+  "Run HOOK with the specified positions BEG and END.
+HOOK should be a symbol, a hook variable.
+The functions are passed integer positions.
+If a function changes the buffer contents, the next function will be
+called with updated positions."
+  (let ((beg-marker (unless (markerp beg) (copy-marker beg)))
+        (end-marker (unless (markerp end) (copy-marker end))))
+    (run-hook-wrapped hook
+                      (lambda (f beg end)
+                        (funcall f (marker-position beg) (marker-position end))
+                        nil)
+                      (or beg-marker beg) (or end-marker end))
+    (when beg-marker (set-marker beg-marker nil))
+    (when end-marker (set-marker end-marker nil))))
+
+(defun edit-indirect--abort ()
+  "Abort indirect edit."
+  (edit-indirect--clean-up))
+
+(defun edit-indirect--clean-up ()
+  "Clean up an edit-indirect buffer."
+  (delete-overlay edit-indirect--overlay)
+  ;; Kill the overlay reference so that `edit-indirect--abort-on-kill-buffer'
+  ;; won't try to call us again.
+  (setq edit-indirect--overlay nil)
+  ;; If we created a window, get rid of it. Kill the buffer we created.
+  (if edit-indirect--should-quit-window
+      (quit-window t)
+    (kill-buffer)))
+
+(defun edit-indirect--rebind-save-hooks ()
+  "Bind our `save-buffer' hooks in the current buffer.
+Does nothing if the current buffer is not an edit-indirect buffer."
+  (when (edit-indirect-buffer-indirect-p)
+    (setq buffer-offer-save t)
+    (add-hook 'write-contents-functions #'edit-indirect--commit-on-save nil t)))
+
+(defun edit-indirect--commit-on-save ()
+  "Commit the indirect edit.
+Should only be called from `write-contents-functions'."
+  (edit-indirect--commit)
+  t)
+
+(defun edit-indirect--abort-on-kill-buffer ()
+  "Abort indirect edit.
+Should be called only from `kill-buffer-hook'."
+  (when edit-indirect--overlay
+    (edit-indirect--abort)))
+
+(defun edit-indirect--barf-if-not-indirect ()
+  "Signal an error if the current buffer is not an edit-indirect buffer.
+The error signaled is `edit-indirect-not-indirect'."
+  (unless edit-indirect--overlay
+    (signal 'edit-indirect-not-indirect '())))
+
+(provide 'edit-indirect)
+;;; edit-indirect.el ends here